The present invention relates to the circuits for use with a user implementable dimmer, and in particular to a power saving arrangement for use with a user implementable phase cut dimmer, wherein the presence or absence of a dimmer is detected and a minimum hold current element is enabled or disabled respectively responsive to the detection.
Solid state lighting, and in particular light emitting diodes (LEDs) are rapidly coming into wide use for lighting applications. In most general lighting applications the LEDs are supplied in one or more strings of serially connected LEDs, thus sharing a common current.
LEDs providing high luminance exhibit a range of forward voltage drops, denoted Vf, and their luminance is primarily a function of current. Brightness control of the LEDs may be performed by either pulse width modulation (PWM) or by amplitude modulation. In a PWM brightness control a fixed current is driven through the LED string, and the duty cycle of the fixed current is adjusted in order to control the LED string brightness. In amplitude modulation the amount of current through the LED string is varied directly, thus adjusting the brightness. LED strings exhibit a particular voltage to current relationship, wherein for a voltage below a minimum operating voltage no appreciable current flows, and for voltages exceeding the minimum operating voltage the current follows an exponential curve responsive to the voltage.
A phase cut dimmer is a device arranged to provide control of the brightness of lighting source by blocking a portion of the alternating current (AC) mains power sine wave from reaching the lighting source. Both leading edge dimmers, wherein the leading edge of the sine wave is blocked by a settable conduction angle, and trailing edge dimmers wherein a trailing edge of the sine wave is blocked, are commercially available. Other phase cut dimmers which allow selection of the portion of the sine wave to pass are also known. Phase cut dimmers are typically implemented by thyristors which require a minimum holding current, denoted Ih to operate smoothly, as described inter alia in U.S. Patent Application Publication S/N 2008/0258647 published Oct. 23, 2008 to Scianna, the entire contents of which is incorporated herein by reference. Minimum holding current Ih is also known as the hypostatic current.
A solid state lighting driver may exhibit extremely low currents in certain states of operation, such as a start up state, or a low brightness state, or whenever a sine wave source approaches zero, which may be insufficient to maintain the minimum holding current Ih thus resulting in flicker or reduced brightness when used with phase cut dimmers, particularly pre-installed commercially available phase cut dimmers. It is thus common practice to provide some sort of dummy load in parallel with the solid state lighting driver to ensure provision of minimum holding current Ih, however the existence of such a dummy load wastes energy, particularly in the event that the dummy load is provided when no dimmer is attached.